[From  The  American  Journal  of  Science,  Vol.  XXXIII,  June,  1912.] 


NOTE  on  MEASUREMENTS  of  RADIO-ACTIV¬ 
ITY  BY  MEANS  OF  ALPHA  RAYS. 

By  W.  R.  Barss. 


(Contributions  from  the  Sloane  Physical  Labor atory, 
Yale  University,  New  Haven,  Conn*,  U.  S*  A*) 


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546  W.  R.  Barss — Measurements  of  Radio-activity. 


Art.  XLY. — Note  on  Measurements  of  Radio-activity  by 
means  of  Alpha  Rays  ;  by  W.  R.  Barss. 

It  is  a  well  known  fact  that  in  a  gas  ionized  by  a-particles 
a  saturation  current  is  obtained  only  when  a  much  larger  poten¬ 
tial  gradient  is  applied  between  the  plates  of  the  ionization 
chamber  than  is  necessary  when  /3 -  or  X-rays  are  the  ionizing 
agents.  Bragg  and  Kleeman*  showed  that  a  current  through  a 
gas,  ionized  by  a-particles,  was  still  unsaturated  when  calculation 
showed  that  the  number  of  ions  lost  by  general  recombination 
was  small.  The  effect  was  ascribed  to  “  Initial  Recombina¬ 
tion  ”  ;  i.e.,  to  some  of  the  ions  being  but  partially  separated 
from  their  parent  molecules  by  the  action  of  the  a-particles. 
In  the  absence  of  an  external  electric  field  these  ions  fall  back 
on  their  parent  molecules  and  are  thus  neutralized.  An  intense 
electric  field  is  supposed  to  complete  the  separation  of  the  ions 
and  to  produce  saturation.  On  this  hypothesis,  lack  of  satura¬ 
tion  would  not  depend  on  the  size  or  shape  of  the  ionization 
vessel  and  saturation  would  be  more  easily  obtained  under 
diminished  pressure. 

Kleemanf  has  shown  that  lack  of  saturation  with  weak  ioni¬ 
zation  by  a-particles  is  not  due  to  diffusion  of  the  ions,  nor 
does  it  depend  on  the  recombination  coefficient.  He  has 
shown  that  “  Initial  Recombination  ”  is  very  small  in  gases 
ionized  by  /3-,  y-  and  X-rays ;  in  other  words,  these  ionizing 
agents  effect  a  more  complete  separation  of  negative  ions  from 
their  parent  molecules. 

MoulinJ  has  proposed  an  explanation  of  the  mechanism  of 
ionization  by  a-particles  as  follows.  The  ions  formed  by  the 
a-particles  are  not  distributed  uniformly  throughout  the  gas, 
but  each  a-particle  has,  associated  with  it,  a  column  of  ions,  the 
axis  of  the  column  being  along  the  path  of  the  particle.  Lack 
of  saturation  is  explained  by  recombination  of  ions  of  opposite 
sign  within  each  column.  This  recombination  between  ions  of 
the  same  column  ought  to  exceed  that  which  would  be  obtained 
for  the  same  number  of  ions  distributed  throughout  the  volume 
of  the  gas.  The  amount  of  the  recombination  between  ions  of 
the  same  column  should  be  much  greater  when  the  field  is 
applied  in  a  direction  parallel  to  the  direction  of  the  column, 
than  when  it  is  applied  in  a  direction  perpendicular  to  it;  for 
the  parallel  field  would  leave  the  columns  intact,  while  the 
perpendicular  field  would  break  each  column  into  two  parts  by 

*  Phil.  Mag.,  xi,  p.  466,  1906. 

f  Phil.  Mag.,  xii,  p.  273,  1906. 

\  Le  Radium,  May,  1908,  p.  136. 


4  W.  R.  JBarss — JMeasurements  of  Radio-activity .  547 

separating  the  positive  and  negative  ions.  Hence  the  lack  of 
saturation  should  he  more  apparent  in  the  former  case  than  in 
the  latter.  These  facts  were  experimentally  determined  by 
Moulin.  He  obtained  saturation  for  the  parallel  held  at  1200 
to  1500  volts  per  centimeter  while  for  the  perpendicular  held 
only  about  200  volts  per  centimeter  were  necessary. 

Moulin  concludes  that  general  recombination  within  the 
columns  (proportional  to  the  square  of  the  density  of  ioniza¬ 
tion  within  the  columns)  is  so  much  greater  than  “Initial  Re¬ 
combination”  that  the  latter  is  negligible  in  comparison. 

Ionization  by  a-particles  was  further  investigated  by  Whee- 
lock;*  among  other  results,  he  obtained  the  following.  When 
an  electric  held  is  applied  parallel  to  the  path  of  the  a-parti- 
cle  and  therefore  parallel  to  the  axis  of  the  column  of  ions,  the 
column  would  not  be  broken  up  and  the  recombination  occur¬ 
ring  would  be  between  ions  belonging  to  the  same  column. 
Since  each  particle  makes  the  same  number  of  ions  along  its 
path,  the  density  of  ionization  would  be  the  same  in  any  one 
column  and  therefore  it  would  be  expected  that  the  ratio  of 
currents  obtained  with  sources  of  different  intensities  would  be 
constant  for  different  potential  gradients  applied.  When  the 
held  is  applied  perpendicular  to  the  column  and  when  the 
source  of  ionization  is  small,  very  few  columns  would  exist  in 
the  ionization  vessel  during  the  time  required  for  the  ions  to 
be  carried  over  to  their  respective  electrodes.  Hence  there 
would  be  little  chance  for  recombination  between  the  columns, 
so  that  it  would  be  expected  that  the  ratio  of  currents  obtained 
with  sources  of  different  intensities  would  be  constant  as  in  the 
case  of  the  parallel  held.  When  the  held  is  perpendicular 
and  the  source  of  ionization  is  stronger,  enough  columns  might 
exist  between  the  electrodes  at  one  time  to  make  recombina¬ 
tion  possible,  not  only  between  ions  of  the  same  column  but 
between  those  of  different  columns.  In  this  case  the  ratio  of 
currents  obtained  with  different  source  intensities  might  not 
be  constant  because  of  the  added  recombination  of  ions  of  dif¬ 
ferent  columns. 

Wheelock  found  that  the  ratio  of  the  current  produced  by  a 
more  intense  source  of  rays  to  that  produced  by  a  weaker 
source  is  constant  for  the  parallel  held ;  that  it  is  approxi¬ 
mately  so  for  the  perpendicular  held  when  the  sources  are 
both  w^eak,  and  that  it  increases  slightly  with  the  potential 
gradient  applied  when  the  sources  are  stronger.  This  is  as 
would  be  expected  if  the  ions  formed  by  a-particles  are  arranged 
in  columns. 

When  the  gas  is  ionized  by  /3-  or  X-rays  it  would  not  be 
expected  that  the  ratio  of  currents  obtained  for  different  source 
*This  Journal,  xxx,  233,  1910. 


548  W.  R.  Barss — Measurements  of  Radio-activity. 

intensities  would  be  constant.  Here  the  ions  are  distributed 
throughout  the  volume  of  the  gas,  and  general  recombination, 
which  depends  upon  the  ionization  density,  i.e.,  upon  the 
number  of  ions  per  cubic  centimeter  in  the  gas,  would  increase 
as  the  ionization  itself  is  increased,  unless  a  saturating  held  is 
applied. 

In  a  great  number  of  important  investigations  in  the  subject 
of  radio-activity,  it  has  been  assumed  that  the  quantity  of 
radio-active  material  present  was  proportional  to  the  ionization 
currents  produced  by  the  a-rays.  In  these  experiments,  elec¬ 
trical  helds  have  been  applied  which  would  have  been  ample 
to  cause  saturation  if  the  ionization  had  been  produced  by 
/3-  or  X-rays,  but  which  are  now  known  to  be  quite  inadequate 
to  produce  saturation  when  a-rays  are  employed.  Results 
which  have  been  obtained  in  this  way  are  of  fundamental 
importance  in  the  theory  of  radio-active  transformation.  They 
include  the  determination  of  relative  quantities  of  radio-active 
substances  by  the  “  Emanation  Method  ”  and  the  method  of 
thin  films,  as  well  as  nearly  all  the  measurements  of  rates  of 
decay  of  such  substances.  It  is  safe  to  say  that  in  no  case  in 
which  such  measurements  have  been  made  with  an  electroscope, 
in  air  at  atmospheric  pressure,  has  a  saturating  potential  been 
applied,  or  even  very  closely  approached.  The  fact  that  a 
fairly  consistent  body  of  measurements  and  constants  has  been 
built  up  by  many  investigators,  notwithstanding  this  apparent 
flaw  in  their  experimental  arrangements,  shows  that  the  con¬ 
siderations  advanced  above  must  have  a  considerable  degree  of 
validity.  The  object  of  the  present  experiments  is  to  test  this 
point  specifically  in  the  important  case  when  the  a-rays  are 
produced  by  an  emanation  mixed  with  the  ionized  gas.  In 
this  case  the  sources  of  the  rays  are  scattered  through  the  gas 
and  on  the  walls  of  the  vessel,  and  the  paths  of  the  a-particles 
and  their  attending  columns  of  ions  extend  in  all  directions  ; 
so  that  the  geometrical  complication  is  as  great  as  it  can 
well  be. 

We  might  reasonably  expect  the  ratio  of  currents  to  be  con¬ 
stant  in  this  case,  at  least  for  small  source  intensities.  If  the 
number  of  a-particles  is  small,  there  will  be  only  a  few  columns 
of  ions  existing  together  during  the  time  required  for  the  ions 
to  be  carried  to  their  respective  electrodes.  It  is  true  that  a 
portion  of  the  a-particles  will  cross  each  -  other  and  that  the 
separated  columns  of  ions  will  also  sometimes  cross  each  other, 
thus  producing  some  recombination  between  ions  of  different 
columns.  But  even  when  this  happens,  the  crossing  will  usu¬ 
ally  be  at  an  angle,  and  the  length  of  each  column  is  so  great 
compared  with  the  diameter  of  its  cross  section  that  even  if 
they  do  intersect,  the  amount  of  this  recombination  will  be  neg- 


W.  R.  Barss — Measurements  of  Radio-activity.  549 

ligible  compared  to  the  recombination  between  ions  of  the 
same  column. 

As  the  intensity  of  the  source  is  increased,  the  number  of  col¬ 
umns  of  ions  existing  together  is  also  increased.  The  proba¬ 
bility  that  different  columns  will  cross  each  other  is  greater  and 
therefore  the  amount  of  recombination  between  ions  of  dif¬ 
ferent  columns  will  be  greater.  So  that,  as  in  the  case  of  the 
perpendicular  field,  the  ratio  of  the  larger  current  to  the  smaller- 
wili  probably  increase  as  the  potential  gradient  is  increased. 

In  the  present  experiments,  a  cylindrical  tin  chamber  was 
used  13*5cm  high  and  10-5cra  in  diameter.  A  central  brass 


Fig.  1. 


electrode,  provided  with  an  earthed  guard  ring,  was  connected 
to  a  tilted  electroscope  of  the  Wilson  type,  the  leaf  of  which 
was  observed  by  means  of  a  microscope  having  a  graduated 
scale  in  the  eyepiece.  This  central  electrode  and  the  leaf  of 
the  electroscope  were  grounded  through  a  potentiometer  by 
means  of  which  each  deflection  due  to  the  ionization  current 
was  calibrated  in  terms  of  potential.  The  capacity  of  the  sys¬ 
tem  was  kept  constant,  so  that  these  calibrated  readings  varied 
directly  as  the  actual  ionization  currents.  Different  potentials 
were  applied  to  the  case  of  the  chamber.  Radium  emanation 
was  used  as  an  ionizing  agent ;  it  has  a  half  value  period  of 
about  four  days,  so  that  it  provided  a  suitable  source  of  varying 
intensity. 

One  series  of  observed  data  is  given  in  the  following  table 
Y  is  the  potential  in  volts  applied  to  the  case.  C,  represents 
the  corresponding  ionization  current  for  a  given  intensity  ;  C2 
the  ionization  current  for  a  weaker  intensity,  etc. 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XXXIII,  No.  198. — June,’  1912. 


550  W.  R.  Barss — Measurements  of  Radio-activity . 


V 

Cl 

C2 

c3 

c4 

c6 

2 

•72 

•55 

•40 

•23 

•16 

4 

1-60 

1*15 

•85 

•50 

•38 

8 

2-50 

1-90 

1-40 

•80 

*55 

16 

2-90 

2-20 

1-50 

•96 

*65 

40 

3*35 

2*40 

1*75 

1*05 

•75 

80 

3*53 

2-58 

1-86 

1*10 

•78 

200 

3*65 

2*80 

1-95 

1*16 

•80 

400 

3-80 

2*90 

2-05 

•83 

600 

3*90 

3*00 

2*08 

A  series  of  curves  plotted  from  these  data  is  given  in  fig.  1: 
abscissse  represent  the  potential  Y  applied  to  the  case.  Curve 
1  has  for  its  ordinates  the  values  given  in  C,  above,  curve  2  the 
values  in  C2,  etc. 

Ratios  of  ionization  currents  are  given  in  the  following  table: 


V 

Cx/C2 

Cx/C3 

Cx/C4 

Cx/Co 

2 

1-31 

1*80 

3*13 

4*50 

4 

1-39 

1-88 

3-20 

4*21 

8 

1-31 

1*80 

3*12 

4*54 

16 

1-32 

1-93 

3*02 

4*45 

40 

1*39 

1*91 

3-19 

4*46 

80 

1*36 

1*89 

3*21 

4-52 

200 

1-30 

1-87 

3-14 

4*56 

400 

1*31 

1-85 

4*57 

600 

1-30 

1*87 

:  is  evident  that  the 

current  ratios 

are  constant 

within 

limits  of  experimental  error. 

In  the  above  data  the  potential  applied  to  the  case  was  nega¬ 
tive.  A  series  of  readings  was  made  with  the  potential  posi¬ 
tive  giving  similar  results. 

The  radium  emanation  used  was  drawn  from  carnotite, 
the  amount  of  emanation  being  equivalent  to  the  amount  in 
equilibrium  with  about  10“8  gm.  of  radium.  It  remains  to 
be  tried  to  what  degree  the  intensity  may  be  increased  before 
there  is  a  change  in  the  current  ratios. 

Summary. 

When  the  a-particles  are  moving  in  all  directions  with 
respect  to  the  electric  field,  and  when  the  source  of  ionization 
is  not  too  intense,  the  ratio  of  the  currents  obtained  from  two 
sources  of  different  intensities  is  constant  for  different  poten¬ 
tials  applied  to  the  ionization  chamber. 

Ro  great  errors  are  involved  even  when  currents  are  used 
less  than  one-fifth  of  the  saturation  value. 

In  conclusion,  I  want  to  thank  Professor  Bumstead  for  his 
many  suggestions  throughout  the  experiment. 

Sloane  Physical  Laboratory,  Yale  University,  New  Haven,  Conn. 


